1. Field of the Invention
The present invention relates to a semiconductor laser module suitable for use in optical communication systems or in optical information processing systems, and particularly relates to a semiconductor laser module for optically connecting an optical fiber with a laser beam emitted from a semiconductor laser, the temperature of which is controlled by an electronic temperature control device.
This application is based on Patent Application No. Hei 9-044756 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
A semiconductor laser module is widely used as a signal light source in various optical fiber transmission systems. High reliability is required in these optical communication systems; hence the semiconductor laser module used in these systems is required to provide high reliability for long periods.
FIG. 2 shows a structure of a conventional semiconductor laser module described in Japanese Patent Application, First Publication, No. Hei 5-150146. FIG. 2A shows the side view, and FIG. 2B shows the front view of a conventional laser module.
Referring to FIGS. 2A and 2B, the conventional semiconductor laser module 10 is encapsulated in an module package 11, and an end portion of an optical fiber 12 for transmitting the light signal is fixed by YAG (Yttrium Aluminium Garnet) laser welding through a side wall 11F of the module package 11. A groove 11M is formed in the inner bottom 11B of the module package 11 in a direction parallel to the laser-light axis and an electronic cooling device 13 is fixed by soldering in the groove 11M. An electronic cooling device 13 is composed of a plurality of unit cooling elements 13M held in between an upper insulating plate 13U and a lower insulating plate 13D, and a plurality of unit cooling elements is connected and fixed by soldering with a thin-film or thick-film metal conductor patterns formed on both inner surfaces of the upper and lower insulating plate. A base plate 14 is provided with perpendicular lateral edge portions 14a and 14b at both side edges which are arranged in a direction parallel to the light axis, and the base plate 14 is attached by soldering on the upper insulating plate 13U of the electronic cooling device 13. A support plate 14S on which the semiconductor laser 15 is mounted is attached on the top surface of the base plate 14 by soldering. A lens 16 is mounted on the base plate 14 by YAG laser welding for optically connecting the light beam emitted from the semiconductor laser with the optical fiber.
However, the conventional semiconductor laser module illustrated in FIG. 2 has a problem in that it does not have a long-term reliability, because optical components such as the semiconductor laser and the lens are displaced by creeping of soldered portions and consequently these optical components forming the light axis from the semiconductor laser to the optical fiber through the lens become misaligned with the original light axis. The reasons of displacements will be described below.
As shown in FIGS. 2A and 2B, in the conventional semiconductor laser module 10, an end portion of the optical fiber 12 is fixed by YAG laser welding through the side wall 11F of the module package 11. The electronic cooling device 13 is mounted by soldering on the inner bottom surface 11B of the module package 11. Abase plate 14 is attached on the top surface of the upper plate of the electronic cooling device 13 by soldering. The semiconductor laser 15 is mounted by soldering on the top surface of the base plate 14, and the lens 16 is also mounted by YAG laser welding on the base plate 14.
The lens 16 is located for aligning its light axis with the direction of the laser beam emitted from the semiconductor laser 15 such that the semiconductor laser is optically connected with the optical fiber 12. The optical fiber 12 is located at the same height as that of the light emitting portion of the semiconductor laser 15.
In the above conventional semiconductor laser module 10, the long-term reliability is deteriorated by creeping of soldered portions, such as, portions between the base plate 14 and the electronic cooling device 13, and between the electronic cooling device 13 and the module package. When creeping occurs, the base plate 14 or the electronic cooling device 13 is displaced, and the semiconductor laser 15 and the lens 16 mounted on the base plate 14 are displaced from their original positions on the light axis between the semiconductor laser 15 and the optical fiber 12 through the lens 16.
However, owing to perpendicular lateral portions 14a and 14b of the base plate 14, it is possible to avoid displacements of the base plate 14 as well as the semiconductor laser 15 and the lens 16 from their original positions as far as in a direction orthogonal to the light axis.
Similarly, the groove 11M formed in a parallel direction to the light axis formed in the inner bottom 11B of the module package 11 may prevent the soldered portion between the module package 11 and the lower plate 13B of the electronic cooling device 13 from being displaced as far as in a direction orthogonal to the light axis.
However, it is not possible for the conventional semiconductor laser module to be prevented from being displaced in the direction parallel to the light axis.
Furthermore, in the case when the module package is mounted in an system vertically, namely, in the direction parallel to the direction of gravity, the weight of both base plate and electronic cooling device is applied directly to the soldered portions. In such an arrangement, soldered portions may be more liable to creep than if the module package 11 were arranged horizontally.
As described above, long-term reliability therefore cannot be expected for the conventional semiconductor laser module.